Estimation of Deformation Modulus of Azad Pumped Storage Powerhouse Cavern Using Back Analysis Based on Combination of Extensometer and Load Cell Results

Most back analysis techniques in geotechnical problems are based on methods that utilize the monitored data of stress, strain and displacement. In this research, by performing the back analysis on cavern of Azad pumped storage powerhouse, Iran, deformation modulus of rock mass has been estimated using displacement and load data. First, after analyzing the instrumentation results, the data recorded in different instrumentation stations have been compared to each other. Afterward, using 2D finite element software Phase 2, the modeling was carried out and verified by extensometer results. Then, by performing back analysis using direct method, and comparing modeling results to the results obtained by instrumentation, input parameters of the model, especially deformation modulus of rock mass, have been estimated. Review and combination of extensometer and load cell results indicate that the results are optimally consistent, and also, there is a relationship with high correlation between the applied load to anchor and the rock mass deformation. The difference of less than 10% between the displacement recorded by the extensometer and the equivalent displacement obtained by load cell proved that using load cell in the back analysis is a suitable and cost-effective method. In order to evaluate and minimize the difference between the measured values and the numerical modeling results, statistical indices and error functions are employed. According to the examinations and using extensometer and load cell results, carrying out the back analysis with a deformation modulus of 15.4 GPa is of higher consistency with instrumentation results.